Method of manufacture of heavy metallic material



Dec. 12, 1939. c. J. SMITHELLS METHOD OF MANUFACTURE OF HEAVY METALLICMATERIAL Filed March 22, 1939 17.9 NICKEL WEIGHT% INVENTOR AMES SM/THLLS ATTORNEY Patented Dec. 12, 1939 METHOD OF MANUFAC PATENT OFFICETURE OF HEAVY METALLIC MATERIAL Colin James .Smithells,

or to The General El Iondon, England Application March 22, In GreatBrita 6 Claims.

This invention relates to themanufacture of the new metallic material,described in my pending application No. 220,209, which is composed oftungsten, nickel and copper, has a density of not less than 16 gm./ml.,and consists of large grains of tungsten cemented together by atungsten-nickel-copper alloy, there being not more than 1500 grains oftungsten per square millimetre of section of the material.

The object of the invention is to provide a process, which is amodification of that described in British patent specification No.447,567, for producing the said material by means of which a givendensity of the material may cheaply be obtained.

The invention will 11 and reference will drawing in which Figure 1 showscurve of the new material to its density,

Figures 2 and 3 show microscopic views of etched sections of knownmetallic materials produced by the process described in the said Britishpatent specification No. 447,567 and Figure 4 shows a microscopic viewof an etched section of an example of the new metallic material producedby the method in accordance with the invention.

In the process described in the said British patent specification acompressed mixture of fine tungsten powder and fine powder of nickeland/or copper is heated to a temperature not exceeding 1500" C.

If in carrying out this process the starting materials consist oftungsten and copper powders only, without nickel, the resulting metallicmaterial always consists of the original tungsten particles, not greatlyincreased in size and preserving their original angular shape, cementedtogether by copper, which does not fill all the space between theparticles.

This structure is illustrated in Figure 2; the dimension marked acorresponds to about 0.11 mm. In this figure, I are tungsten grains, 2the cementing copper, 3 voids. Such a structure is to be expected; forit is known that molten copper, while it wets tungsten, does not alloywith it at temperatures below 1500 C.

On the other hand, if the starting mixture contains nickel, which whenmolten dissolves tungsten, the tungsten particles in the resulting alloytend to be substantially larger and more nearly spherical than theoriginal particles. Apparently the small tungsten particles dissolve inthe 6 nickel and the larger particles grow by taking ow be described indetail be made to the accompanying 20 s relating the composition Rugby,England, assignectric Company Limited,

1939, Serial No. 263,388 in June 24, 1938 up tungsten from the solution.But if nickel is present without copper, the growth of the tungstenparticles is apt to be limited, at least if they are not originally verysmall; the nickel (in which is dissolved some tungsten) is apt tocollect in large pools and not to penetrate between the tungstenparticles. This structure is illustrated in Figure 3, where thedimension marked u again corresponds to about 0.11 mm. Here I are thetungsten grains, 4 the nickel pools, 3 the voids. 10

I have now found that if both nickel and copper are present in suitableproportions and coarse tungsten powder is used in the starting mixture,the growth of the tungsten particles is much more pronounced and themolten nickel-copper-tungl6 sten alloy penetrates freely between them.This effect of the copper is probably due to its reducing the meltingpoint (especially when the sintering temperature is near the meltingpoint of nickel) and surface tension of the molten material.

Thus in my method I make an intimate mix-- ture of nickel powder, copperpowder and coarse tungsten powder; I form from this mixture by means ofpressure a self-supporting body and I then sinter this body until thenew metallic material is formed.

The resulting new metallic material is a com plex of nearly sphericaltungsten grains, much larger than the original particles, cementedtogether by a nickel-copper-tungsten alloy, almost without voids. Thisstructure is illustrated in Figure 4, where the dimension marked a isagain about 0.11 mm. Here I are the large tungsten grains, 5 thenickel-copper-tungsten alloy filling completely the spaces between them.

Structures intermediate those of the known metallic material shown inFigures 2 and 3 and the new metallic material shown in Figure 4 may beproduced. Thus, if the time or temperature of sintering is too small, astructure intermediate between that of Figures 2 and 4 may be produced,even if both nickel and copper are present. With the new material thetheoretical density is attained more nearly than with the knownmaterials having structures as shown in Figures 2 and 3. While it ispossible to attain, or approach very nearly, the theoretical density ina material containing no copper by reducing the voids in a structure ofthe kind shown in Figure 3 by increasing the time and temperature of thesintering it is much easier and cheaper to attain the theoreticaldensity, or a given fraction of it, with the new material produced bythe method in accordance with the invention.

The theoretical density means the density calculated from theproportions and densities of the components (that is to say, 19.3 fortungsten, 8.9 for copper or nickel) on the assumption that there are novoids and no interpenetration. Since no interpenetration appears ever tooccur, the theoretical density is the maximum density that a material ofgiven composition can attain. Of course every theoretical density is notgreate. than any non-theoretical density; for the theoretical densitydepends on the composition. Thus a material containing 12% of(copper+nickel) which has the theoretical density 16.9 is less densethan one containing 4% of (copper+nickel-) and having only 91% of thetheoretical density 18.4. Nevertheless it is desirable that metallicmaterials of the kind referred to should have as nearly as possible thetheoretical density; for at a given density, deficiency from thetheoretical density means more tungsten (which is the expensiveconstituent) and also usually less desirable mechanical properties.

Materials that contain copper without nickel, and more than 83%tungsten, never attain substantially theoretical density, however greatthe proportion of copper and however long the period of sintering; forit appears that voids can be filled up only by the process of solutionand growth that leads to the large-grain structure. On the other handmaterials containing nickel and copper in suflicient quantity, and inproportions adapted to promote the formation of the largegrain structurecharacteristic of the new material produced by the method in accordancewith the invention, attain quickly densities 95% of the theoretical, andin practicable conditions may attain 98%.

The limits of composition within which the new material can be producedby the method in accordance with the invention depend mainly on threefactors, namely (1) the grain size of the coarse tungsten powder, (2)the temperature at which the mixture is sintered, (3) the time for whichit is sintered. The pressure by which the material is compressed beforesintering is relatively unimportant, so long as it is sufficient to forma self-supporting body; 5 tons per sq. in. has alwaysproved sufficient.The self-supporting body shrinks greatly (e. g., about 20% in lineardimensions) during sintering. Here it is to be noted that, in themanufacture of the new material by this process, the temperature mustnever be so high that the body collapses and loses its shape; this isimplied by the term sintering. The grain size of the copper and nickelis also unimportant, solong as it is not much greater than that of thetungsten. The surrounding atmosphere during sintering must be neutralor, preferably, reducing; but there appears to be no difference betweenpure hydrogen and a nonexplosive mixture of hydrogen and nitrogen.

I will now proceed to describe with reference to Figure 1, the factsregarding composition for one particular choice of the factors (1), (2),(3) In the experiments to which this figure refers, the tungsten powderhas 35% of its particles less than 2a in diameter and substantially nonegreater than 8; in diameter; such a powder is considerably coarser thanthat now generally used as a starting material for making tungsten lampfilaments, and is considerably cheaper.

The mixtures were all sintered at 1450 C. for one hour. It will berealised by those skilled in the art that this account of the grain sizedoes not identify the powder completely; but it is diiflcult to describethe grain size in any manner that does identify it completely.Accordingly, since the time and temperature of heating necessary toobtain a given result varies with the grain size, it is not assertedthat the results about to be described will be obtained with any powderhaving a grain size within the said limits. All that is essential isthat there is a powder, hav-- ing a grain size within these limits, fromwhich the results about to be described can be obtained by heating to1450 C. for one hour; and that the results can be obtained from anypowder having a grain size within these limits, if the time andtemperature of heating are adjusted to a value not very different fromthose given. Here it may be observed that it is not easy to measure afurnace temperature of 1450" C. by commercial methods .with an errormuch less than 25 C.; accordingly 50 C. is not to be regarded as a largedifference.

In the figure the abscissae are the percentages of nickel, the ordinatesthe percentages of copper; the remainder is always tungsten.(Percentages, here and everywhere, are percentages by weight.) Thestraight lines crossing the diagram from left-top to bottom-rightdenote, by the numerals marked against them, the theoretical densitiesof the compositions through which they pass. The dotted lines denote, bythe numerals marked against them, the actual densities of thecompositions through which they pass. Compositions not on a dotted 'linegive, of course, densities intermediate between those marked on thedotted lines between which they lie; those inside the line marked 17.0have densities not less than that figure.

I have found that all those compositions that lie on or inside the linemarked 16.0 have the characteristic structure of the new metallicmaterial.

It will be seen from the diagram that (1) The new metallic materialalmost always has substantially more nickel than copper; the nickelcontent always lies between 4 and 11% and the copper content between 1and 6%.

(2) With 5% of nickel and 5% of copper the new material just has adensity of 16; with 5% of nickel and no copper thematerial produced bythe process described has a density of less than 14 and has not thestructure characteristic of the new metallic material.

(3) The new metallic material almost always has a density less than thetheoretical; but, when the proportion of copper and nickel in an alloyis suitable, the deficiency may be only 2%.

(4) The maximum density is in the neighbourhood of 6% nickel and 3%copper.

However it should be noted that density per se may not always be theonly consideration. Tensile strength and ease of machining may beimportant. These qualities are determined almost wholly by density andare practically independent of composition, density being constant.Machinability decreases generally as density increases, so that acompromise may have to be made. But the variation is not large. The newmaterial of density 16 has a Brinell hardness of some 220, that ofdensity 1'7 a hardness of 290; both can be machined without seriousdifficulty. The composition that my experiments indicate to be the bestfor most purposes is 6.4% nickel and 2.6% copper.

The statements just made with reference to the figure are true only ifthe grain size, temperature and time of heating are as aforesaid. I

will now explain how they are to be modified if these factors arechanged.

It may be said broadly that decrease of particle size, increase oftemperature, and increase of time of heating are equivalent, and thatthe effect of any one of them or of all in combination is to increasethe area enclosed within any given dotted curve, (and so the range ofcompositions within which any given density can be obtained) subject, ofcourse, to the condition that a curve can never cross the straight linesimilarly marked. With extension of the area corresponding to densitiesabove a given limit goes, in general, extension of the area within whichthe new material may be obtained.

Again, if the grain size to which the figure refers is used, but thetemperature increased to 1500 C. for 1 hour, the area within which thenew material may be obtained is again increased. For, since the finalequilibrium is attained more nearly, the new material will attain morenearly the theoretical density.

Increase of time produces much the same eifect as increase oftemperature; but the increase has to be considerable. Thus an increaseof the time of sintering from 1 to 6 hours is approximately equivalentto an increase of temperature from 1450 C. to 1500 C.

Conversely, a diagram very similar to Figure 1, though not entirelyidentical with it, would result from decreasing the particle size anddecreasing also the time or temperature of sintering. If the powder isfine enough and the time of sintering long enough, such a diagram couldbe produced at a temperature as low as 1400 C., or even lower.

On the other hand if much coarser powder was used, considerably greatertemperatures or times would be necessary to obtain the new material. Theparticle size of the powder to which the figure refers is not far fromthe coarsest that can be used economically.

Considerable enlargement of the curves of the figure is not practicallydesirable. For finer powder, higher temperature and longer period ofsintering all increase the cost of manufacture; while the rise in themaximum density obtainable is not nearly so marked as increase in therange of compositions over which a given density, and especially a veryhigh density such as 17, can be obtained. It is therefore desirable tolimit the fineness of the powder, the temperature and the time ofheating. In virtue of what has been said such a limit can be imposed inpractice by limiting the compositions of the new material; for, thoughit is possible to use more expensive methods than are actuallynecessary, these are not likely to be employed. The limits chosen may besomewhat wider than those indicated in the figure; for a slightenlargement of the curves does not involve any considerable extraexpense. The limits of composition may then be taken to be -13 ofcopper, 3 -16 of nickel, and

83-96% of tungsten.

I claim:

1. The method of manufacturing a new metallic material composed oftungsten, nickel and copper, said material having a density of not lessthan 16 gm./ml. and consisting of large grains of tungsten cementedtogether by a tungstennickel-copper alloy, there being not more than1500 grains of tungsten per square millimetre of section of thematerial, which comprises the processes of forming an intimate mixtureof nickel powder, copper powder and coarse tungsten powder, thecomposition by weight of the mixture lying within the range 3 -16 ofnickel, -13V of copper and 83-96% of tungsten, subjecting said mixtureto pressure so as to form a self-supporting body and sintering said bodyuntil said new metallic material is formed.

2. The method of manufacturing a new metallic material composed oftungsten, nickel and copper, said material having a density of not lessthan 16 gin/ml. and consisting of large grains of tungsten cementedtogether by a. tungstennickel-copper alloy, there being not more than1500 grains of tungsten per square millimeter of section of thematerial, which comprises the processes of forming an intimate mixtureof nickel powder, copper powder and coarse tungsten powder, thecomposition by weight of the mixture lying within the range 3 -16 ofnickel, -13 of copper and 83-96% of tungsten, and wherein not more than35% of the tungsten grains in the mixture are less than 2p. indiamsupporting body and sintering said body until said new metallicmaterial is formed.

3. The method of manufacturing a new metallic material composed oftungsten, nickel and copper, said material having a density of not lessthan 16 gm./ml. and consisting of large grains of tungsten cementedtogether by a tungstennickel-copper alloy, there being not more than 11500 grains of tungsten per square millimetre of section of thematerial, which comprises the processes of forming an intimate mixtureof nickel powder, copper powder and coarse tungsten powder, thecomposition by weight of the mixture lying within the range 3V216/2% ofnickel, V213!/2% of copper and 83-96% of tungsten, and wherein not morethan 35% of the tungsten grains in the mixture are less than 2 indiameter and the remainder of the tungsten grains are between 2 and 8 indiameter, subjecting said mixture to pressure so as to form aself-supporting body and sintering said body at about 1450" C. for about1 hour.

4. The method of manufacturing a new metallic material composed oftungsten, nickel and copper, said material having a density of not lessthan 16 gm./ml. and consisting of large grains of tungsten cementedtogether by a tungstennickel-copper alloy, there being not more than1500 grains of tungsten per square millimetre of section of thematerial, which comprises the processes of forming an intimate mixtureof nickel powder, copper powder and coarse tungsten powder, thecomposition by weight of the mixture being 6.4% of nickel, 2.6% ofcopper and 91% of tungsten, subjecting said mixture to pressure so as toform a self-supporting body and sintering said body until said newmetallic material is formed.

5. The method of manufacturing a new metallic material composed oftungsten, nickel and copper, said material having a density of not lessthan 16 gm./ml. and consisting of large grains of tungsten cementedtogether by a tungstennickel-copper alloy, there being not more than1500 grains of tungsten per square millimetre of section of thematerial, which comprises the processes of forming an intimate mixtureof nickel powder, copper powder and coarse tungsten powder, thecomposition by weight of the mixture being 6.4% nickel, 2.6% of copperand 91% of tungsten, and wherein not more than 35% of the tungstengrains in the mixture are less than 2 in diameter and the remainder ofthe tungsten grains are between 2p and 8; in diameter, subjecting saidmixture to pressure so as to form a self-supporting body and sinteringsaid body until said new metallic material is formed.

6. The method of manufacturing a new metallic material composed oftungsten, nickel and copper, said material having a density of not lessthan 16 gm./ml. and consisting of large grains of tungsten cementedtogether by a tungstennickei-copper alloy, there being not more than1500 grains of tungsten per square millimetre of section of thematerial, which comprises the processes of forming an intimate mixtureof nickel powder, copper powder and coarse tungsten powder, thecomposition by weight of the mixture being 6.4% of nickel, 2.6% ofcopper and 91% of tungsten, and wherein not more than 35% of thetungsten grains in the mixture are less than 2p. in diameter and theremainder of the tungsten grains are between 2p and 8 in diameter,subjecting said mixture to pressure so as to form a self-supporting bodyand sintering said body at 10 about 1450 C. for about 1 hour.

COLIN JA MES SmTI-IELIS.

